1. Field of the Invention
The present invention is a system and method for using alignment marks on a substrate for alignment with a photolithographic system.
2. Background of the Invention
Alignment tools, such as photolithographic systems, typically do a global search to provide a coarse alignment of a substrate that is loaded into the system. This coarse alignment locates alignment marks within a specified tolerance and removes large translation and rotation errors in the placement of the substrate in the system before a finer grained alignment is performed for the substrate. Removal of these potentially large errors during coarse alignment enables fine alignment systems to efficiently capture alignment marks on the substrate during the fine alignment process.
Fine alignment systems generally use one of three types of detection methods: bright field, dark field and diffraction pattern. Of these methods, diffraction pattern detection tends to have the smallest capture range during fine alignment. This small capture range is due to the repetition of signal peaks, which are used to identify fine alignment position, in the diffraction of an alignment source signal incident on the diffraction pattern of an alignment mark. The diffraction pattern signal received at a signal detector of the lithographic system has multiple peaks, as shown in FIG. 1A to FIG. 1C. These multiple peaks can cause confusion in determining the proper aligned position of the substrate. The confusion is due to the fact that the multiple peaks appear identical to the alignment system. The capture range of the primary diffracted signal is equal to +/−½ the period of the diffraction pattern signal. Thus, exceeding this capture range during a search for the proper aligned position of the substrate can lead to large errors in substrate placement and undermine the proper operation of a fine alignment system.
FIG. 1A to FIG. 1C show examples of background art diffraction pattern signals used during coarse alignment. FIG. 1A shows a diffraction pattern signal with a periodicity of 4 μm. This periodicity provides a capture range of +/−2_μμμm for the coarse alignment of a substrate. FIG. 1B shows a diffraction pattern signal with a periodicity of 5 μm. This periodicity provides a capture range of +/−2.5_μm for the coarse alignment of a substrate. The capture range can be extended by using a diffraction pattern signal that is the sum of diffraction pattern signals, as shown in FIG. 1C. The periodicity of the combined diffraction pattern signals provides an extended capture range of +/−10 μm, as shown in FIG. 1C. Though the approach of FIG. 1C does extend the coarse capture range, further improvement in the performance of alignment systems could be provided by even larger increases in the coarse capture range.
In addition, background art diffraction detection systems use either: (1) a completely separate alignment system that locates the fine alignment marks to less than +/− the fine alignment capture range; or (2) a diffraction alignment system that determines the fine alignment to allowable tolerances using two diffraction gratings of different frequencies. However, the need exists for an alignment technique that increases the coarse capture range and still properly aligns the substrate. Therefore, there is a need for improved coarse capture range in the art of alignment systems.